This application incorporates by reference Taiwan application Serial No. 090110666, filed May 3, 2001.
1. Field of the Invention
The invention relates in general to a sheet feeding apparatus, and in particular, to a sheet feeding apparatus using an elastic device.
2. Description of the Related Art
The performance of the conventional image forming apparatus, such as a printer, copy machine, facsimile machine, or the like, is greatly affected by the quality of sheet feeding. A sheet feeding apparatus capable of separating and feeding sheets one-by-one smoothly and continuously is therefore in great demand.
Taking the printer as an example of the image forming apparatus, a conventional sheet feeding apparatus is illustrated in FIG. 1A. The printer is typically equipped with an automatic sheet feeding apparatus, wherein sheets of papers are first positioned at a rest plate and driven one-by-one by the sheet feeding apparatus to the image forming apparatus. In FIG. 1A, a base 101 for supporting the sheets is at the bottom of the automatic sheet feeding apparatus. A roller 150 is installed above the base 101 to drive the sheets forward. The base 101 has a sheet-separating pad 103, for separating sheets one-by-one, and a number of smooth ribs 102.
The front view shown in FIG. 1B illustrates the movement of the sheets. The sheet separating pad 103 and the smooth ribs 102 are located on the base 101 of the automatic sheet feeding apparatus and are parallel to each other. The ribs 102 act to elevate the sheets in order to facilitate the smooth movement of the individual sheets. Materials with low friction coefficient are ideal for the manufacture of the ribs 102; thus a smooth rubber or plastic is frequently used because of the additional advantage of low cost. The side view shown in FIG. 1C illustrates the movement of the sheets. The sheet 105 is driven by the roller 150 (not shown), and is moved along the direction A. The front edge 107 of the sheet 105 touches the surfaces of the ribs 102 and slides along the direction A.
Then referring to FIG. 1D, the friction force to the sheet is illustrated. While the sheet 105 is forwarded along the direction A, the ribs 102 generate a friction force against the movement of the sheet 105. If the thickness of the sheet 105 reaches to a certain level, the friction force will not have much influence on the movement of the sheet. However, if the sheet 105 is thin, the lower edge 107 can bend around, instead of moving forward smoothly, as shown in FIG. 1E.
In FIG. 1E, the lower edge 107 of sheet 105 is in contact with the ribs 102. The roller (not shown) drives the sheet 105 forward along the direction A, but the friction force pushes the sheet backward along the direction B. As the roller continues to drive the sheet 105, the sheet 105 can potentially roll up and cause a paper jam.
In addition to the ribs 102, the sheet-separating pad 103 is also a key element of the conventional base 101. FIG. 2 shows the side view of the sheet-separating pad 103 and the sheets 105. The sheet-separating pad 103 is shaped similarly to a saw in that it has a number of sawtooth-like protrusions 106. Each protrusion 106 rises up at an angle xcex1. The friction force generated by the sheet-separating pad 103 against the sheets is large enough to separate each sheet. The sheet-separating pad 103 is typically positioned along the sheet moving direction A. Thus, as the sheet 105 moves and rises along the sawtooth-like protrusions 106, it is separated from the other sheets. The movement of the sheet 105 depends on the angle xcex1 and the friction coefficient, which is affected by the material and thickness of the sheet 105.
The type of printing sheet can vary greatly, with the material and thickness being just two factors. The thickness, usually defined by g/m2, can vary within a wide range, for example, from thick postcards, envelopes, and plastic plates to thin plain papers. The conventional sheet-separating pad is designed to satisfy a certain range of printing materials. The incline angle xcex1 and the material type (which affects the coefficient of friction) of the sheet separating pad 103 are determined factors, thus only printing materials within the designated range are suitable. Printing materials outside the designated range may cause paper jams or reduce the sheet feeding accuracy. Furthermore, the high precision requirement in the manufacturing of the sheet-separating pad causes difficulties in fabrication and results in higher cost. To summarize, the disadvantages of the conventional sheet feeding apparatus comprise the following:
1. paper jams;
2. limited printing materials;
3. less accuracy in sheet separation; and
4. higher cost in fabrication of the sheet-separating pad.
It is therefore an object of the invention to provide a sheet feeding apparatus that enhances the stability of sheet separation and reduces the potential for paper jams.
According to the object of the invention, a sheet-separating pad for use in a sheet feeding apparatus is disclosed. The sheet-separating pad comprises a pad base having a top surface and a groove. Numerous inclined protrusions are formed at one or two sides of the groove on the top surface of the base to guide the sheet(s) in a certain direction. An elastic device, such as a spiral spring or a series of connected spring leaves, is situated in the groove. The spiral spring or series of spring leaves protrudes above the top surface of the base in order to separate the sheets. Driven by rollers, the sheets rise gradually along the protrusions so that the sheets can be separated one-by-one. Moreover, the radial arrangement of the ribs and the sheet-separating pad allows the lower edge of each sheet to move forward smoothly such that a paper jam can be prevented.